Unpredictable rock formations and small boulders caused major delays during the tunnel construction for Namma Metro Phase 1. Now, this can be avoided with the help of advanced seismic tomography developed by the National Institute of Rock Mechanics (NIRM).

Engineers from Bangalore Metro Rail Corporation Limited (BMRCL) urged the NIRM to come up with a technology that can help in understanding underground rock formations in the built-up surface areas in Phase 2.

NIRM director H S Venkatesh said it was the only institute in India with a specialised geophysics engineering group that can provide the precise information on the location and size of the boulder. “Several years of experience has helped the team of experts to come up with sophisticated technology that can help avoid delay in projects,” he said.

BMRCL officials said while huge boulders pose a challenge in tunnelling work, tunnel boring machines (TBM) can cut through them in a time-consuming process. Smaller boulders of 3-5 metre in diameter start rotating along with the blades of the TBM, instead of getting cut.

Metro engineers said the damage to the cutter-head of the Godavari TBM in April 2014 stalled the work for over a year. Importing a new cutter-head from Italy and delay in the work led to more than
Rs 10 crore extra.

Dr P C Jha, scientist and the head of Department of Geophysics at NIRM, said they have the technology to map the boulders. He said BMRCL contractors use borehole logs to assess the underground surface, which will not be able to map the boulder.

“The seismic refraction study, done from the surface, can tell the type of the rock and its location within 20-metre reach. However, if a free surface is not available along the alignment, NIRM’s seismic tomography survey (which is done from boreholes) can provide a full projection of the size of the boulder besides the usual information. Though it is 4-5 times costlier than the surface survey, it is negligible considering the cost of delay,” he said.